Liquefied natural gas (LNG) is principally liquid methane with smaller amounts of C2+ hydrocarbons also present. LNG is prepared by chilling a raw natural gas stream to a temperature and at a pressure sufficient to cause at least a portion of the methane in the raw gas to condense as a liquid. The natural gas stream from which the LNG is made may be recovered from any process which generates light hydrocarbon gases. However, generally the raw natural gas from which LNG is prepared is recovered from a crude oil or gas well.
Raw natural gas is a mixture of various hydrocarbon gases, including C2− hydrocarbons, and heavier C3 and C4 petroleum gases. “Wet” gas also comprises varying amounts of C5+ hydrocarbons, while “dry” gas comprises little or no C5+ hydrocarbons. As used herein, C1 represents a hydrocarbonaceous compound having one carbon atom per molecule, C2 contains two carbon atoms per molecule, etc. C3-C4 represents a hydrocarbonaceous material comprising compounds having three carbon atoms per molecule and/or compounds having four carbon atoms per molecule. C3+ compounds represents compounds having three or more carbon atoms per molecule. C5+ represents compounds having five or more carbon atoms per molecule. Methane is a representative example of a C1 compound. Ethane, ethylene, and mixtures thereof are representative examples of C2 compounds. Propane, propene, butane, butenes and mixtures thereof are representative examples of C3-C4 compounds. Pentanes, pentenes, hexanes, hexenes and comparable higher molecular weight species, and their mixtures, are representative of C5+ compounds.
The process of liquefying natural gas involves chilling the raw natural gas, either at atmospheric or super-atmospheric pressure, until the methane and ethane condense as liquids. On account of their higher molecular weights and lower dew points, any C3+ vapors contained in the raw natural gas condense prior to the condensation of the C1 and C2 compounds, forming a liquid product termed “natural gas liquids” which may be abbreviated as “NGL”. Each of the components which condense during the preparation of LNG have important commercial value. C1 and C2 compounds are the major components of LNG and are valuable both as fuel and as feedstock for preparing commercially valuable products. Liquefied petroleum gas (LPG), comprising principally C3 and C4 hydrocarbons, is useful as a refrigerant in the liquefaction process. LPG also may serve as a fuel in the LNG liquefaction process or as transportation or heating fuel. The C5+ condensate recovered from the raw natural gases is valuable as a blending component for fuels, particularly for transportation fuels. Therefore, it is important that the liquefied C5+ condensate and the C3-C4 LPG be recovered separately from the LNG. The present invention is directed to an efficient process for recovering and storing separate LPG streams in the process of preparing LNG.
As used in this disclosure the words “comprises” or “comprising” are intended as open-ended transitions meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrases “consists essentially of” or “consisting essentially of” are intended to mean the exclusion of other elements of any essential significance to the composition. The phrases “consisting of” or “consists of” are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.